I. Technical Field
The present invention relates generally to a shutdown and recovery routine for an automatic transmission, and more particularly, a shutdown and recovery routine which allows an automatic transmission to shift between an increased number of gear states.
II. Discussion
In the present design of automatic transmissions, a series of clutches or friction elements provide a means for application and release of separate members to and from each other during the flow of power through the transmission. These clutches thereby constitute the means by which gears within the transmission are selectively engaged or disengaged from either the engine crankshaft or the transmission case. Four speed transmissions, of the type which this invention is directed, generally include any number of elements or clutches which are applied or engaged in various combinations in relation to each of the vehicle's gears. Each of these clutches generally includes a plurality of alternating clutch plates and clutch disks which, when applied, engage one another and which disengage when the clutch is not applied
To apply each of these clutches, an electronically controlled hydraulic fluid actuating device such as a solenoid-actuated valve is used. There is typically one valve for each clutch. These valves each control fluid flow to a respective clutch apply cavity. The flow of fluid into a clutch apply cavity results in the application or engagement of that clutch. Fluid flow is enabled by the opening of the solenoid-actuated valve in response to command or control signals received by the solenoid from an electronic control system.
The electronic control system typically includes a microcontroller-based electronic control system capable of receiving input signals indicative of various vehicle operating conditions such as engine speed, torque converter turbine speed, transmission output shaft speed (vehicle speed), throttle angle position, brake application, predetermined hydraulic pressures, a driver selected gear or operating condition (PRND2L), engine coolant temperature and/or the ambient air temperature. Based on the information contained in these signals, the controller generates command or control signals for causing the actuation of each of the solenoid-actuated valves which regulate the application and release of fluid pressure to and from the apply cavities of the clutches or frictional units of the transmission. In most transmissions, the torque converter input speed and the output shaft speed are used to determine which gear state the transmission is presently operating. This information can then be used to ensure that the actual transmission gear state matches the gear state indicated in the shift schedule. Accordingly, the controller is programmed to execute predetermined shift schedules stored in a memory of the controller through appropriate command signals to the solenoid-actuated valves.
However, if the electronic control system reports a different gear state, based on the torque converter and output speed, than is reported by the shift schedule in the processor, then one or the other is providing incorrect information indicative of a defective sensor or other device. If the electronic control system attempts to change gear states under these conditions, the improper readings may cause the automatic transmission to engage or disengage improper clutch elements. This could result in damage to the transmission or injury to a vehicle driver. If the electronic control system maintains the transmission in its present gear state, not allowing gear change, the vehicle is forced to operate over a broad range of speeds in only one gear. Thus, the driver experiences poor driving characteristics to the point of possibly not being able to operate the vehicle. The present invention was developed in light of these drawbacks.